No, that's not quite true. The PS5 is not limited to 1080p - depending on the connection, it can also output resolutions such as 1440p or even 4K, provided the monitor supports this.
However, with a 34-inch ultrawide monitor with 3440 × 1440, you won't get a true 21:9 image. Instead, a 16:9 signal is usually displayed, either with black bars on the left and right or stretched by the monitor to the full width.